Technology update

Nov 3, 2008

CNT loudspeaker rips up the design book

A transparent carbon nanotube (CNT) thin film developed by Lin Xiao and colleagues at Tsinghua University, China, could turn out to be a wonder material for makers of audio visual devices. The see-through structure emits sound when hooked up to an electrical signal and can be stretched over a display to play audio content (see video 1 and video 2) eliminating the need for conventional loudspeakers or headphones.

To emit sound, the device relies on the so-called thermoacoustic effect. In other words, it is the thermal expansion and contraction of air in the vicinity of the thin film (due to the periodic heating of the CNTs) that produces sound, not the mechanical movement of the thin film itself.

Because the thin film itself does not vibrate, the loudspeaker will continue to work even if part of the film is broken or if the device is mounted on soft materials such as flags or clothing

The scientists found papers published more than 100 years ago describing platinum wires or foils that could emit sound in a similar way, but the output was described to be very weak. Analysis performed by the team shows that the sound pressure generated by a one-layer CNT loudspeaker should be considerably greater than the output produced by a 700 nm thick platinum film. This is due to the much lower specific heat capacity per unit area of the CNT thin film compared with the platinum structure.

The group's device also benefits from the use of super-aligned carbon nanotube arrays, which are easy to process into a continuous thin film or ribbon (see related story).

"We can batch-synthesize super-aligned CNT arrays onto 4 inch silicon wafers, which each provide enough material for a continuous thin film that can be up to 60 m long and up to 10 cm wide," the researchers told nanotechweb.org. "Because the thin film itself does not vibrate, the loudspeaker will continue to work even if part of the film is broken or if the device is mounted on soft materials such as flags or clothing."